Conductor casting apparatus for a squirrel-cage rotor of an induction motor

ABSTRACT

A rotor core (12) of a squirrel-cage rotor of an induction motor is housed in a recess (14) of a mold (18) of a conductor casting apparatus (10). A composite cylinder device (68) is fixed on the upper side of the mold (18) or on the side remoter from the pouring gate thereof, and the rotor core 12 is held in a predetermined position in the recess 14 by one piston (70). The other piston (72) locally pressurizes a molten metal that fills an end-connector ring cavity (66) defined between the rotor core (12) and the recess (14) on the side remoter from the pouring gate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuous application of International PatentApplication No. PCT/JP 96/01326, filed May 20, 1996,

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuous application of International PatentApplication No. PCT/JP 96/01326, filed May 20, 1996,

TECHNICAL FIELD

The present invention relates to a conductor casting apparatus formolding a conductor of a squirrel-cage rotor of an induction motor to beintegrally with a rotor core, and more particularly, to a conductorcasting apparatus provided with means for additionally pressurizing amolten metal that fills an end-connector ring cavity between a mold anda rotor core on the side remoter from the pouring gate of the mold, inorder to eliminate blowholes formed in a molded conductor.

BACKGROUND ART

As shown in FIGS. 3A and 3B, a rotor core 12 of a squirrel-cage rotorused in an induction motor is formed of a laminate of magnetic sheets,such as silicon steel sheets, and comprises a plurality of conductorrods 46 arranged individually in a plurality of slots 44 that are formedin the axial direction along the substantially cylindrical outerperipheral edge, and a pair of end-connector rings 48 which are arrangedindividually at the axially opposite ends of the core 12 toshort-circuit the conductor rods 46 one another.

These conductor rods 46 and the pair of end-connector rings 48 areintegrally molded by casting, such as aluminum die casting. Acylindrical axial hole 50 in the center of the rotor core 12 which isintegral with molded conductor portion is fixed on a shaft 52 by shrinkfitting or the like, whereupon the well-known squirrel-cage rotor isformed.

This conductor molding method is excellent in productivity andfacilitates molding of a conductor in a desired shape, so that it has anadvantage of being able to improve the characteristics of the motor.Heretofore, it has been widely used for a small-sized induction motors,in particular.

Conductor molding by casting, however, may bring on a problem thatshrinkage cavity or gas cavity is caused in a molded conductor.According to the die casing method in which a molten metal, such asaluminum, is loaded at high speed and under high pressure, inparticular, the molten metal is liable to form a turbulent flow and trapgas as it is loaded, so that development of blowholes is unavoidable. Inthe squirrel-cage rotor, moreover, the radial sectional area of eachconductor rod is much smaller than the radial sectional area of eachend-connector ring. During the casting operation, therefore, the moltenmetal first solidifies in the slots (in which the conductor rods arearranged to form the squirrel-cage rotor) that axially penetrate thelaminate of magnetic sheets, such as silicon steel sheets, so that asufficient pressure cannot be transmitted to those end-connector ringportions which solidify after a delay (especially those end-connectorring portions which are located on the side remoter from the pouringgate), and blowholes tend to be easily formed. Since a conductor havingblowholes is low in mechanical strength, it is difficult to apply asquirrel-cage rotor including such a conductor to a high-torqueinduction motor or high-speed induction motor with a speed of tens ofthousands of rpm.

Already proposed is a casting method in which a molten metal filling anend-connector ring cavity between a mold and a rotor core is locallypressurized independently of the filling pressure, in order to preventthe formation of blowholes in the conductor (end-connector ring inparticular). According to this method, the molten metal is loaded fromone end-connector ring cavity, which communicates with a pouring gate,into the other end-connector ring cavity, on the side remoter from thepouring gate, through a plurality of slots, and first solidifies in theslots with a small sectional area, thereby cutting off applied pressurefrom the end-connector ring cavity on the side remoter from the pouringgate. As this is done, pressure is additionally applied to the moltenmetal in the end-connector ring cavity on the side remoter from thepouring gate before this molten metal solidifies. Thus, theend-connector ring on the side remoter from the pouring gate can bemolded under a desired pressure, so that the formation of blowholes canbe prevented.

This local-pressurization casting method requires use of localpressurization means that can apply a uniform pressure to the wholeend-connector ring on the side remoter from the pouring gate. For easeof manufacture, however, many molds for squirrel-cage rotors aregenerally constructed so as to be able to open and close in the samedirection as the axial direction of the rotor core. In installing thelocal pressurization means on the side remoter from the pouring gate ofone such mold, the installation space for the local pressurization meansmust be provided in the mold so as not to influence external structures,such as a mold supporting structure. Thus, the mold is increased inthickness and external dimensions, so that its manufacture is difficult,and its practicability is ruined.

Further, the molds for squirrel-cage rotors must be provided with aretaining structure for firmly holding the rotor core in a predeterminedposition in the mold, resisting the molten metal filling pressure, onthe side remoter from the pouring gate. How to arrange this retainingstructure, together with the aforesaid local pressuring means, orderlyon the side remoter from the pouring gate of the mold is also a problemto be solved.

Also proposed is a local pressurization method in which the gate-sideportion of the mold is designed so as to be movable in the axialdirection, and the movable pouring gate-side portion of the mold isaxially moved together with the rotor core by the filling pressure afterthe molten metal is loaded from the pouring gate-side end-connector ringcavity into the end-connector ring cavity on the side remoter from thepouring gate through the slots, whereby pressure is additionally appliedto the end-connector ring on the side remoter from the pouring gate.Although the local pressurization means need not be located on the sideremoter from the pouring gate, according to this method, theconstruction of the mold is complicated, and moreover, a special castingmachine (injection molding machine) with a very high pressure is neededto move a movable part of the mold together with the rotor core, so thatthe installation cost is very high.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a conductor castingapparatus, which is provided with local pressurization means capable ofpreventing a conductor, especially an end-connector ring on the sideremoter from the pouring gate, from suffering blowholes by using acasting machine with an ordinary injection pressure, without increasingthe external dimensions of a mold or complicating the structure, wherebya high-strength, high-performance squirrel-cage rotor that is applicableto a high-torque or high-speed induction motor can be manufactured.

In order to achieve the above object, a conductor casting apparatus fora squirrel-cage rotor of an induction motor according to the presentinvention, which is used to mold a plurality of axially extendingconductor rods and a pair of end-connector rings for short-circuitingthe respective axially opposite ends of the rods, to be integral with arotor core by casting, is arranged so that its mold has a recesscontaining the rotor core and a pouring gate, communicating with a firstend-connector ring cavity defined between the wall of the recess and oneaxial-end face of the rotor core, and adapted to open and close in theradial direction with respect to the rotor core, and its compositecylinder device includes a first piston located adjacent to a secondend-connector ring cavity, defined between the wall surface of therecess and the other axial end face of the rotor core on the sideremoter from the pouring gate, and fixedly holding the rotor core in therecess, and a second piston for additionally pressurizing a molten metalfilling the second end-connector ring cavity, the first and secondpistons being arranged concentrically for independent actuation.

According to the present invention arranged in this manner, the firstpiston of the composite cylinder device firmly fixedly holds the rotorcore in the recess of the mold against the molten metal fillingpressure, during casting operation. The molten metal supplied to thefirst end-connector ring cavity through the pouring gate flows throughthe rotor core and reaches the second end-connector ring cavity. Beforeit solidifies in the second end-connector ring cavity, the molten metalsolidifies in the rotor core, thereby forming the conductor rods inadvance and cutting off applied pressure from the second end-connectorring cavity. Then, the second piston of the composite cylinder device isactuated to pressurize the molten metal in the second end-connector ringcavity additionally. Thereupon, the molten metal solidifies under adesired pressure, so that end-connector rings free from blowholes areformed.

Since the mold opens and closes in the radial direction with respect tothe rotor core, an installation space for the composite cylinder devicecan be easily secured on that side of the mold which is remoter from thepouring gate. Since the first and second pistons are arrangedconcentrically for independent actuation, moreover, there is nopossibility of the thickness and external dimensions of the mold beingincreased or the structure being complicated. Since the molten metalfilling pressure is not used to move the mold or the core, furthermore,a casting machine with an ordinary injection pressure can be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front sectional view showing a conductor casting apparatusaccording to an embodiment of the present invention with its mold open;

FIG. 2 is a side view, partially in section, taken along line II--II,showing the conductor casting apparatus with a rotor core housed in arecess;

FIG. 3A is a front view of a squirrel-cage rotor manufactured by usingthe conductor casting apparatus of FIG. 1;

FIG. 3B is a side sectional view of the squirrel-cage rotor taken alongline b--b of FIG. 3A;

FIG. 4 is a front sectional view showing the rotor core of thesquirrel-cage rotor of FIGS. 3A and 3B fixed to a jig;

FIG. 5 is a partial enlarged side sectional view showing a compositecylinder device of the conductor casting apparatus of FIG. 1; and

FIG. 6 is a side sectional view of a modification of the compositecylinder device of FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIGS. 1 and 2, there is shown one embodiment of a conductorcasting apparatus for a squirrel-cage rotor of an induction motoraccording to the present invention.

The conductor casting apparatus 10 comprises a mold 18 provided with arecess 14, which houses a rotor core 12 of the squirrel-cage rotor, anda pouring gate 16 that communicates with the recess 14. The recess 14and the pouring gate 16 are connected by means of a sprue runner 20 thatdiverges substantially symmetrically in two directions. The mold 18 isdivided into two parts, that is, a stationary part 24 and a movable part26, by parting faces 22 that extend in the vertical (longitudinal)direction. The movable part 26 can reciprocate in the horizontal(crosswise) direction (arrow A) with respect to the stationary part 24.Thus, the mold 18 is constructed so as to open and close in the radialdirection with respect to the rotor core 12.

The recess 14, pouring gate 16, and sprue runner 20 are cut in thestationary part 24 and the movable part 26 from the parting faces 22 sothat they have substantially the same shape, and form one mold cavityand a sprue runner system when the two parts 24 and 26 are combinedtogether. In the present embodiment, the stationary part 24 and themovable part 26 are formed of outside main molds 24a and 26a and insidesub-molds (liners) 24b and 26b, respectively, and the recess 14, pouringgate 16, and sprue runner 20 are formed in the sub-molds 24b and 26b.

The pouring gate 16 is located right under the recess 14, and the spruerunner 20 opens in the peripheral wall of the lower end of the recess14. Underlying the pouring gate 16, a receiving section 28, which is tobe fitted with the injection tip of a casting machine, such as a diecasting machine, is formed in the respective main molds 24a and 26a ofthe stationary part 24 and the movable part 26. Further, a plurality ofgas vents 30 for discharging gas from the cavity during castingoperation are formed in the stationary part 24 and the movable part 26so as to extend along the parting faces 22. The gas vents 30 penetratethe main molds 24a and 26a and the sub-molds 24b and 26b, and open inthe peripheral wall of the upper end of the recess 14 and the respectiveouter surfaces of the main molds 24a and 26a.

The movable part 26 is provided with a plurality of guide pins 32projecting from the parting face 22 of the main mold 26a and a coresupporting rail 34 projecting from the parting face 22 of the sub-mold26b. The stationary part 24 is provided with guide grooves 36 and a railreceiving groove 38 that are formed recessed from the parting face 22 inpositions corresponding to the guide pins 32 and the core supportingrail 34, respectively. Further, the movable part 26 is furnished with anextruding plate 40 and a plurality of extruding pins 42 for extrudingthe core with a molded conductor portion from the recess 14.

The rotor core 12 (see FIGS. 3A and 3B) of the squirrel-cage rotor,which is formed of a laminate of magnetic sheets, is stationed in therecess 14 of the mold 18 so as to be fixedly supported by a jig 54 shownin FIG. 4. The jig 54 is provided with a mandrel section 56, which isinserted in an axial hole 50 of the rotor core 12, and a pair offastening sections 58 and 60 connected individually to the axiallyopposite ends of the mandrel section 56 by fixing means such as bolts(not shown) and integrally holding the rotor core 12. These fasteningsections 58 and 60 include substantially cylindrical small-diameterportions 58a and 60a, inserted in the axial hole 50 and in contact withthe mandrel section,56, and substantially cylindrical large-diameterportions 58b and 60b, extending from the small-diameter portions 58a and60a and engaging the axially opposite end faces of the rotor core 12,respectively. The outside diameter of the large-diameter portions 58band 60b is smaller than the diameter of a circle that connects therespective radially outermost edges of slots 44 of the rotor core 12 andlarger than the diameter of the diameter of the axial hole 50.

The rotor core 12, which is fixedly supported by the jig 54, is placedon the core supporting rail 34 of the movable part 26 in a manner suchthat its axis is directed vertically, as shown in FIG. 1. The coresupporting rail 34 is formed with a pair of ridges 62 (FIG. 2) forguiding the rotor core 12 into the recess 14, and the lower fasteningsection 58 slidably engages the ridges 62. In the embodiment of FIG. 4,the fastening section 58 is provided with a second small-diameterportion 58c that extends further from the large-diameter portion 58b,and this small-diameter portion 58c engages the pair of ridges 62 of thecore supporting rail 34. The axial end face of the small-diameterportion 58c of the fastening section 58 and the axial end face of thelarge-diameter portion 60b of the upper fastening section 60 aresubstantially flat surfaces such that the rotor core 12 can be fixedtight in the mold 18.

The rotor core 12, fixedly supported by the jig 54, is inserted into therecess 14 of the movable part 26, and the movable part 26 is moved andattached to the stationary part 24. Thereupon, the rotor core 12 ishoused in position with its outer peripheral surface in substantiallyuniformly intimate contact with the peripheral wall of the recess 14. Inthis state, a first annular end-connector ring cavity 64 (FIG. 2) formolding, an end-connector ring 48 is formed, in the lower end region ofthe recess 14, between the wall surface of the recess 14 and the axialend face of the rotor core 12 and the outer peripheral surface of thelarge-diameter portion 58b of the fastening section 58. Likewise, asecond annular end-connector ring cavity 66 (FIG. 2) for molding anotherend-connector ring 48 is formed, in the upper end region of the recess14, between the wall surface of the recess 14 and the axial end face ofthe rotor core 12 and the outer peripheral surface of the large-diameterportion 60b of the fastening section 60. The first and secondend-connector ring cavities 64 and 66 communicate with the slots 44 ofthe rotor core 12.

The conductor casting apparatus 10 according to the present inventioncomprises a composite cylinder device 68, which functions both asretaining means for firmly holding the rotor core 12 in a predeterminedposition in the recess 14 against the molten metal filling pressure, andas local pressurization means for locally pressurizing a molten metal,which fills the conductor molding cavity, independently of the fillingpressure. As shown in FIGS. 1 and 2, the composite cylinder device 68 isfixedly supported on the upper end of the movable part 26 on the sideremoter from the pouring gate, and is provided with a first piston 70for fixing the rotor core 12 in the recess 14 and a second piston 72 foradditionally pressurizing the molten metal filling the secondend-connector ring cavity 66.

As is conceptually shown in FIG. 5, the composite cylinder device 68 hasthe first and second pistons 70 and 72 arranged substantiallyconcentrically for independent actuation. Thus, the composite cylinderdevice 68 is provided with a first chamber 74 containing the firstpiston 70 and a second chamber 76 containing the second piston 72, whichare isolated from each other lest any circulation of a fluid be causedbetween them. A piston rod 78 of the first piston 70 penetrates a pistonrod 80 of the second piston 72 substantially concentrically like atelescope. The first and second chambers 74 and 76 have ports 82 and 84and ports 86 and 88, respectively, outside the respective strokes of thefirst and second pistons 70 and 72, and individually constituteindependent double-acting cylinders. A drive source for the compositecylinder device 68 may be hydraulic or pneumatic.

A push flange 90 having a substantially flat axial end face is attachedto the distal end of the piston rod 78 of the first piston 70. When thefirst piston 70 advances, the axial end face of the push flange 90 abutsagainst the axial end face of the large-diameter portion 60b of thefastening section 60 of the jig 54 that supports the rotor core 12 inthe recess 14 (see FIG. 5). Thus, the rotor core 12 is held firmly inthe predetermined position in the recess 14. A pressure then supplied tothe first chamber 74 through the port 82 is high enough to retain therotor core 12, resisting the molten metal filling pressure.

A hollow cylindrical push ring 92 having a substantially flat annularaxial end face is attached to the distal end of the piston rod 80 of thesecond piston 72. The push ring 92 houses the push flange 90 of thefirst piston 70 substantially concentrically. The axial end face of thepush ring 92 is located opposing the second end-connector ring cavity66. When the second piston 72 retreats, the axial end face of the pushring 92 is located at an axial distance a little greater than the axialdimension of the completed end-connector ring 48 from the axial end faceof the rotor core 12 (see FIG. 5). When the second piston 72 advances,the axial end face of the push ring 93 pressurizes the molten metal inthe second end-connector ring cavity 66 additionally and generallyuniformly. A pressure then supplied to the second chamber 76 through theport 86 is high enough to prevent the molten metal from sufferingblowholes as it solidifies in the second end-connector ring cavity 66.

The following is a description of a procedure for molding the conductorportion to be integral with the rotor core 12 by using the conductorcasting apparatus 10 having the aforementioned construction.

First, the mold 18 is opened, and the first and second pistons 70 and 72of the composite cylinder device 68 are located in their respectiveretreated positions. Then, the rotor core 12, which is fixedly supportedby the jig 54, is placed on the distal end portion of the coresupporting rail 34 of the movable part 26 in a manner such that its axisis directed vertically with the fastening section 58 downward. In thisstate, the movable part 26 is moved and attached to the stationary part24, and the rotor core 12 is loaded into the recess 14. In order tosimplify the procedure, in this case, it is advisable to insert therotor core 12 first into the recess 14 of the stationary part 24 as themovable part 26 moves and cause it to be pressed by the wall of therecess 14 of the stationary part 24 to be guided on the core supportingrail 34 so that the insertion of the movable part 26 into the recess 14is completed when the movable part 26 is attached entirely to thestationary part 24.

After the movable part 26 is mounted on the stationary parts 24 and thenthe rotor core 12 is housed entirely in the recess 14, a given pressureis supplied to the first chamber 74 of the composite cylinder device 68through the port 82, and the first piston 70 is advanced so that therotor core 12 is held firmly between the push flange 90 and the coresupporting rail 34. In this state, the injection tip of a castingmachine such as a die casting is attached to the receiving section 28,and the molten metal is injected into the pouring gate 16. The moltenmetal, injected under a given pressure, flows through the pouring gate16 into the first end-connector ring cavity 64, runs against the gravityin the slots 44 of the rotor core 12, and further flows into the secondend-connector ring cavity 66, thereby entirely filling the conductormolding cavity in the recess 14. As this is done, the gas in theconductor molding cavity is discharged from the mold through the gasvents 30.

Before the pressure is fully transmitted to the molten metal that wasintroduced into the second end-connector connector ring cavity 66, themolten metal that fills the slots 44 of the rotor core 12 starts tosolidify first, thereby cutting off pressure application to the secondend-connector ring cavity 66. After the second end-connector ring cavity66 is entirely filled with the molten metal, therefore, a given pressureis supplied to the second chamber 76 of the composite cylinder device 68through the port 86, and the second piston 72 is advanced so that themolten metal in the second end-connector ring cavity 66 is pressurizedby the push ring 92. Thus, the end-connector ring 48 on the side remoterfrom the pouring gate is also formed under a sufficient pressure, sothat a high-strength conductor portion that is free from gas defects,such as sink marks, pores, etc. The timing for the second piston 72 tobe actuated to pressurize the molten metal in the second end-connectorring cavity 66 can be obtained experimentally on the basis of the moltenmetal filling speed, the respective capacities of the slots and theend-connector ring cavities 64 and 66, the speed of the second piston72, etc.

After the molding of a plurality of conductor rods 46 and the pair ofend-connector rings 48 for the rotor core 12 is thus completed, pressureis supplied to the first and second chambers 74 and 76 of the compositecylinder device 68 through the ports 84 and 88, respectively, whereuponthe first and second pistons 70 and 72 are retreated. Further, themovable part 26 is moved to open the mold 18, and the extruding plate 40is actuated so that the rotor core 12 is extruded toward the distal endportion of the core supporting rail 34 by the extruding pins 42. Then,the jig 54 is disengaged from the rotor core 12, and finishing work,such as removal of flashes from the conductor portion is made to therotor core 12. Thereafter, the rotor core 12 is fixed to a shaft 52,whereupon the squirrel-cage rotor shown in FIG. 3 is formed.

In the conductor casting apparatus 10 according this embodiment, asdescribed above, the composite cylinder device 68 which has coreretaining function and local pressurization function for the moltenmetal is located on the side remoter from the pouring gate of the mold18 that opens and closes in the radial direction with respect to therotor core 12, so that the conductor, especially the end-connector ringon the side remoter from the pouring gate, can be prevented fromsuffering blowholes, and a high-performance squirrel-cage rotor can bemanufactured without increasing the thickness and external dimensions ofthe mold 18 or complicating the structure. Moreover, the necessarymolten metal filling pressure for casting is as high as the pressure forconventional casting that involves no local pressurization, and thecasting machine used can employ an ordinary injection pressure, so thatthe installation cost can be prevented from jumping.

In a mold structure in which molten metal flows in the antigravitydirection from under a cavity into the cavity, as in the mold 18 of theconductor casting apparatus 10, a plurality of independent cavities(e.g., slots 44) can be filled with the molten metal under a uniforminfluence of the gravity, so that the molten metal can be prevented fromflowing ill-balanced and trapping air Thus, this structure is optimum incasting a conductor of a squirrel-cage rotor. Moreover, a mold structurein which the rotor core 12 is housed with its axis directed verticallyand the stationary part and the movable part are separated in thehorizontal direction at their vertical parting faces, is advantageous inthat the installation space for the composite cylinder device accordingto the present invention can be secured with ease, and that the gasvents can be located in optimum positions, as in the case of theaforementioned embodiment.

In the composite cylinder device 68 according to the foregoingembodiment, the pressure to be supplied to the second piston 72 for thelocal pressurization of the molten metal is about five to ten times ashigh as the pressure to be supplied to be to the first piston 70 for theretention of the core. Depending on the diameter of the rotor to becast, therefore, the second piston 72 is much greater in size than theone illustrated, and sometimes is not favorable in practical use. Insuch a case, it is advisable to use a composite cylinder device 94 thathas a second piston of a tandem structure, as shown in FIG. 6.

The composite cylinder device 94, like the composite cylinder device 68,is fixedly supported on the upper end of a mold on the side remoter fromthe pouring gate, and is provided with a first piston 96 for holding arotor core and a second piston 98 for locally pressurizing a moltenmetal. The first and second pistons 96 and 98 are arranged substantiallyconcentrically for independent actuation. A first chamber 100 housingthe first piston 96, a second chamber 104 housing a first piston element102 of the second piston 98, and a third chamber 108 housing a secondpiston element 106 of the second piston 98 are isolated from one anotherlest any circulation of a fluid be caused between them. A piston rod 110of the first piston 96 penetrates a piston rod 112 of the second piston98 substantially concentrically like a telescope. The first, second, andthird chambers 100, 104 and 108 have their respective ports 114 outsidethe respective strokes of the first piston 96 and the first and secondpiston elements 102 and 106 of the second piston 98.

Since the composite cylinder device 94, which is provided with thetandem-type second piston 98, has a core retaining function and also alocal pressurization device for the molten metal, gas defects of aconductor portion can be prevented without increasing the externaldimensions of the mold or complicating the structure. Since the radialdimension of the second piston can be made smaller than that of thesecond piston 72 of FIG. 5, moreover, this arrangement is advantageousparticularly in the case where the conductor casting apparatus issubject to dimensional restrictions.

According to the present invention, as described above, the compositecylinder device, which has core retaining function and also localpressurization function for the molten metal, is incorporated in themold that opens and closes in the radial direction with respect to therotor core. Accordingly, the conductor, especially the end-connectorring on the side remoter from the pouring gate, can be prevented fromsuffering blowholes as it is molded to be integral with the rotor coreby using the casting machine with the ordinary injection pressure,without increasing the thickness and external dimensions of the mold orcomplicating the structure. Thus, a high-strength, high-performancesquirrel-cage rotor that is applicable to a high-torque or high-speedinduction motor can be manufactured at low cost.

We claim:
 1. A conductor casting apparatus for a squirrel-cage rotor ofan induction motor, which is used for molding a conductor comprising aplurality of axially extending conductor rods and a pair ofend-connector rings for short-circuiting the respective axially oppositeends of the rods, to be integral with a rotor core by casting,comprising:a mold having a recess for housing the rotor core and apouring gate, opening into a first end-connector ring cavity definedbetween the wall surface of said recess and one axial end face of therotor core, and adapted to open and close in the radial direction withrespect to the rotor core; and a composite cylinder device including afirst piston located adjacent to a second end-connector ring cavity,defined between the wall surface of said recess and the other axial endface of the rotor core on the side remoter from the pouring gate, andfixedly holding the rotor core in said recess, and a second piston foradditionally pressurizing a molten metal filling the secondend-connector ring cavity, the first and second pistons being arrangedconcentrically for independent actuation.
 2. A conductor castingapparatus according to claim 1, wherein said second piston of saidcomposite cylinder device has a tandem structure composed of a firstpiston element and a second piston element connected to the first pistonelement, the first and second piston elements being housed,respectively, in first and second chambers isolated from one anotherlest any circulation of a fluid be caused between the two.
 3. Aconductor casting apparatus according to claim 1, wherein a jig isfitted in an axial hole of the rotor core housed in the recess of themold, and said first piston fixedly holds the rotor core in the recessby engaging one end face of the jig.
 4. A conductor casting apparatusaccording to claim 3, wherein said jig includes a mandrel section to beinserted in the axial hole of the rotor core and fastening sectionsconnected individually to the opposite ends of the mandrel section andeach having a large-diameter portion in engagement with an axial endface of the rotor core corresponding thereto.
 5. A conductor castingapparatus according to claim 1, wherein gas vents are formed in partingfaces of the mold adapted to open and close in the radial direction withrespect to the rotor core.
 6. A conductor casting apparatus according toclaim 1, wherein one half of said mold, adapted to open and close in theradial direction, is provided, on the pouring-gate side thereof, with acore supporting rail for carrying the rotor core thereon, and the otherhalf of the mold is provided with a rail receiving groove for receivingsaid core supporting rail.